Duct Sizing Question

Dec 08, 2006 6 Replies
R

If I'm using 8" round duct to facilitate my returns, shouldn't the register boots and the takeoffs have the same area as the 8" pipe?



I have the same question with the supply boots as well. If the area of 8" round is



100 square inches, why then are there so many boots with dimensions like:


4 x 10, 6 x 10, 8 x 10 ?



It appears none of these boot openings or takeoffs have adequate cross-sectional areas to facilitate the round duct size I'm running to them. Won't the air volume be limited by the smallest piece in the branch?



I do realize this question is probably stupid, but my brain short circuited on this for some reason.



Thanks in advance for any help.


Richard


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D

were is you air duct calculator?

8" rd duct only has a 220 cfm capability with .10 static so, (2) 6" returns will be the max or you will get whistling

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G

please explain how you go about doing that when a variable speed furnace blower will vary its delivered cfm from 50 to 100%. :)

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J

The duct has an air flow resistance related to it's size and length. It may be legitimate to have a bigger cross section on a long run to improve the air flow even though there is a lower cross section at the end.

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S

You're a PITA, Fish. :) But to answer your question you design for the max air flow, but then you knew that.

IMO, the trick really comes > Steve Scott wrote:

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U

The linked pages below may not be perfect, but might help answer some individuals' questions.

- udarrell

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